Functional Separation of Endosomal Fusion Factors and the Class C Core Vacuole/Endosome Tethering (CORVET) Complex in Endosome Biogenesis

Abstract

Background:The interdependence of the endosomal fusion factors CORVET and Vac1 has not been addressed. Results: CORVET can be separated from other endocytic fusion factors on the basis of ultrastructure, localization, and trafficking. Conclusion: CORVET acts independently of the Vac1 tether and requires activated Rab5 homologs for localization. Significance: Our data reveal a unique role of CORVET in sorting of biosynthetic cargo to the vacuole.

Guanine Nucleotide Exchange Factors (GEFs) Have a Critical but Not Exclusive Role in Organelle Localization of Rab GTPases

“…2A). For Vps21, it was shown recently that two GEFs, Vps9 and Muk1, act redundantly (33,34). In agreement with this, only the dual deletion of both GEFs resulted in redistribution of Vps21 to the ER among…”

CORVET and HOPS tethering complexes–coordinators of endosome and lysosome fusion

“…In agreement with this, it has been found that purified CORVET is able to tether efficiently vacuole-associated endosomes in a Rab5/Vps21-dependent manner both in vitro and in vivo (Balderhaar et al, 2013). Endosomal fusion and fission appear to be crucial for the generation of MVBs and protein sorting as Rab5 mutants are defective in both processes (Cabrera et al, 2013;Nickerson et al, 2012;Russell et al, 2012). Indeed, the generation of an average MVB of a radius of 100 nm that carries up to 50 intraluminal vesicles (Luhtala and Odorizzi, 2004) would require several fusion events to provide sufficient membrane for the vesicle generation.…”

“…Interestingly, all class D genes encode proteins involved in membrane fusion at endosomes (Bowers and Stevens, 2005). Recently, it was shown that the CORVET subunits have a function that is distinct and possibly downstream from Vps45 and Vac1 (Cabrera et al, 2013), which also binds Vps21-GTP (Peterson et al, 1999;Tall et al, 1999). The CORVET subunits Vps3 and Vps8 localize independently of Vac1, Vps45 and Pep12 but still require activated Rab5 proteins for their localization and function (Cabrera et al, 2013).…”

“…Recently, it was shown that the CORVET subunits have a function that is distinct and possibly downstream from Vps45 and Vac1 (Cabrera et al, 2013), which also binds Vps21-GTP (Peterson et al, 1999;Tall et al, 1999). The CORVET subunits Vps3 and Vps8 localize independently of Vac1, Vps45 and Pep12 but still require activated Rab5 proteins for their localization and function (Cabrera et al, 2013). The most plausible interpretation of these results is that CORVET is a tether for the fusion of endosomes with each other, so-called homotypic fusion, whereas Vac1 might be required primarily for the fusion of endocytic vesicles with endosomes, although the possibility of cooperation occurring between the two tethering machineries cannot be excluded.…”

“…The Rab5 GEF protein Rabex5 (yeast Vps9) might be present already on endocytic vesicles before their fusion with endosomes (Carney et al, 2006). In yeast, Vps21 is the main Rab5 homolog, and it is partially complemented by its other yeast homologs Ypt52 and Ypt53 (Cabrera et al, 2013;Horazdovsky et al, 1994;Nickerson et al, 2012;Singer-Krüger et al, 1994). During endosomal maturation, Rab5 is consecutively replaced by Rab7 (Poteryaev et al, 2010;Rink et al, 2005;Vonderheit and Helenius, 2005) (Box 1).…”